MechChem Africa August 2018

Mixed matrix membranes: Promising materials for AMD treatment By Michael ODaramola

In addition, the resins need to be regenerated when exhaust- ed by chemicals and this regeneration can produce sec- ondary pollution and elevate operational costs. On the other hand, the passive treat- ment of AMD relies on biological, geochemical and gravitation processes in natural or constructed wet- land ecosystems. Furthermore, conventional methods can only achieve partial treatment, and they also have the disadvantages

Water is connected to almost everything on earth. In South Africa, the words ‘mining’ and ‘water’ lead one to think about acid mine drainage (AMD). This month we feature an opinion piece by Professor Michael Daramola on membrane technology to treat AMD. The views expressed in this article are his own, informed by experience, literature review and other expert opinion pieces.

A major sanitation and water pol- lution challenge associated with the mining industry is acid mine drainage (AMD), which forms when sulphide rocks are exposed to air and water for prolonged periods. The formation of acidmine drainage is a natural process, but reactions are caused by exposing sulphide- containing rocks to the environment through mining operations and are often catalysed by bacterial activity. The natural process of AMD formation takes close to 15 years in the absence of bacteria, for ferric iron to produce acid, but the presence of bacteria shortens this reaction time. Typical characteristics of AMD are very low pH and high concentrations of metals and sulphates. If left untreated, AMDhas the significant negative environmental impact of mineralisationof affected areas and acidifica- tion of receiving ground and surface waters. The solubilityof transitionmetals is greater in lowpHmedia, henceAMDcarrieswith it high concentrations of metals such as Al, As, and

Mg and other transition metals such as Cu, Zn, Pb, Co,

Mn andCd, depending on the host rock. It enters the aquatic environment uncontrolla- bly, posing a threat to humans, domesticated animals and the ecosystem. Conventional treatment processes: active and passive treatment Active treatment involves neutralising the acidity with alkaline substances such as lime to trigger precipitation of metal hydroxide, which is easily be removed by sedimenta- tion. Furthermore, ion exchange technology –which explores the advantage of oppositely charged pollutants and employs solid resins to remove cations and anions from solutions – has also been proposed as a treatment method. A high metal ion uptake capability of this resinmakes ion exchange an attractive technology, but it is a preferred technology forlowmetalionconcentrationsandbecomes very expensive when dealing with high con- centrations of metal ions in solutions.

of producing sludge, requiring high-energy consumption and frequent maintenance. Therefore, growing global demand for clean water and increasing environmental concerns, warrant the need to search for more sustainable and environmentally friendly technologies for metal ion removal from mining wastewater. There is a great need for water recycling and efficiency of water recyclingwill depend strongly, amongst other factors, on the performance of existing water treatment techniques andprocesses to provide potable and clean water for the use of the human race without posing environ- mental hazards. Potable water coupled with good sanita- tion should be affordable by all, which is the sixth of the sustainable development goals (SDG): Clean water and sanitation for all. Membrane technology In the search for alternative technologies, membrane technology has proven to be a promising option. Membrane technology, due to its easy operation, inexpensiveness, high separation efficiency and low energy consumption, has emerged as a promising substitute to conventional methods for AMD treatment. Amembrane is a thin layer of semi-perme- ablematerial that separates substanceswhen adriving force is appliedacross it ina selective manner.Mechanical strength, thermal stabil- ity and chemical resistance of a membrane form part of the significant characteristics that define a good membrane and they are highly dependent on the properties of the materials of construction. Mostcommonpressure-drivenmembrane processes, which are distinguished by pore sizes, are microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). But pure polymer membranes have poor chemical resistance, poor mechanical

Michael Daramola is an Associate Professor at the School of Chemical and Metallurgical Engineering, University of theWitwatersrand, Johannesburg, South Africa. He is an established researcher, with a sustained record of research productivity and quality research outputs, who has been evaluated and benchmarked by the SouthAfricanNational ResearchFoundation (NRF) rating system (C-rating) recognised by peers in his field.  Furthermore, he is a Chartered Chemical Engineer (CEng) with the Engineering Council of the UK and a RegisteredEngineerwith theCouncil for theRegulation ofEngineeringinNigeria(COREN).Hisresearchfocuses on sustainable energy and clean environment. He also serves as an associate editor of theChemical

Engineering Communications (CEC), an editorial member of the International Journal of Oil, Gas & Coal Technology (IJOGCT), and an associate editorial board member of the Open Chemical Engineering Journal. He is an active member of SAIChE IChemE in Gauteng. Email: Michael.Daramola@wits.ac.za Extracted from: Machodi, M, Daramola, M O (2018); Sodalite and Chitosan based Composite Membrane Materials for Treatment of Metal-containing Wastewater in Mining Operations.

6 ¦ MechChem Africa • August 2018